The present invention relates to the selective, controlled manipulation of polymers and polymer alloys using radiation chemistry, which makes possible the tailoring of polymer properties for a specific intended use. The present invention finds particular application in the orthopedic field, including the formation of medical prosthesis, such as hip, knee, shoulder, and finger implants.
Methods of irradiating polymers are described in U.S. Pat. No. 5,879,400. In general, this patent describes medical prosthesis formed, at least in part, of a melt-irradiated high molecular weight polyethylene. The disclosed melt-irradiation process improves the wear resistance of the polymer, thus addressing the problem of severe adverse effects associated with the use of less wear resistant polymers. U.S. Pat. No. 5,879,400 describes, among other things, heating the polymers to or above the melting point, irradiating the polymer, and cooling the polymer.
International Application No. PCT/US97/02220 (WO 97/29793) also describes the irradiation of polymers that are useful in the orthopedic field. In this application, several methods of increasing the wear characteristics of polymers are described. The application describes, among other things, an irradiation procedure wherein the polymer is irradiated at room temperature or below. Following irradiation, the polymer can be heated to or above the melting temperature to remove any residual free radicals through the process of recombination. The application also describes another irradiation method in which the polymer is pre-heated to a temperature above room temperature, but below the melting temperature, and irradiated. Following irradiation, the polymer may be subsequently melted by heating it to the melting temperature or above to substantially eliminate any detectable free radicals via the process of recombination.
WO 97/29793 also describes methods of irradiating polymers in which the heat generated by the irradiation is sufficient to at least partially melt the polymer, and is described as “adiabatic melting”. “Adiabatic melting” refers to heating induced by radiation, which leads to an increase of the temperature of the polymer with substantially little loss of heat to the surroundings. The application describes an adiabatic melting method in which the polymer is preheated to a temperature below the melting point, then irradiated with enough total dose and at a high enough dose rate to at least partially melt the polymer crystals. Subsequent to this warm-irradiation, the polymer also can be heated to or above the melting temperature such that any residual free radicals are eliminated. The application also describes another irradiation, adiabatic melting method that is similar to the method described above, except that the polymer is provided at room temperature or below.
International Application No. PCT/US99/16070, describes the use of irradiated polymers for hip joints with an extended range of motion. In particular, this application relates to the use of wear resistant irradiated polymers in hip joint prostheses. The wear resistance of the polymers allows for the use of combinations of cup thicknesses and head diameters that result in an extended range of motion as compared with conventional replacement hip joints, the wear resistance of which was unable to support cup thickness and head diameter combinations that allowed for extended ranges of motion.
Other approaches of irradiation are disclosed in U.S. Pat. Nos. 6,281,264; 6,245,276; 6,242,507; 6,228,900; 6,184,265; 6,165,220; and 6,017,975.
Despite the major improvements in the wear resistance of orthopedic prostheses and the design of hip prostheses allowing an improved range of motion, there remains a significant need for further improvements. For example, the irradiation of polymers is known to change the mechanical properties of the polymer. Following irradiation and subsequent melting and annealing, polyethylene polymers exhibit reduced toughness, reduced modulus of elasticity, reduced shear strength and reduced ultimate tensile strength. In the case of hip prostheses, for example, larger head diameters often require the use of thinner liners. The locking mechanisms on these liners (used to lock the liner to metal shells) may fail due to the undesirable changes in the mechanical properties of the polymers following irradiation. The situation is similar in knee prostheses. In knee prostheses, intricate locking mechanism, usually in the form of snap-lock, pegs and pins, are used to stabilize the liners on a metal tray. These locking mechanisms rely on the high shear strength of the polymer used. When irradiated, the adverse effects on the shear strength of the polymer may jeopardize the stability of the liner. Similar problems arise in other types of medical prostheses.
These and other aspects of the invention will become apparent to the skilled artisan in view of the teachings contained herein.